An Account of the Bell Rock Light-House / Including the Details of the Erection and Peculiar Structure of That Edifice; to Which Is Prefixed a Historical View of the Institution and Progress of the Northern Light-Houses

by Robert Stevenson

The Reporters then go on to state, from a review of the several quarry prices, that, for the outer casing, the sum of about L. 2,500 would be saved by the use of sandstone from Mylnefield, instead of granite from Aberdeen; and that, for the hearting of the solid part, an additional saving of about L. 1000 would further be made, if the sandstone of the Redhead quarries, in the immediate neighbourhood of Arbroath, instead of the Mylnefield stone, was used. On considering this subject, however, in all its bearings, the Commissioners resolved that measures should be taken for procuring granite for the whole outward casing of the Light-house, and that the Mylnefield sandstone should be used for the interior work. To the other properties of these stones, one of some consideration for a work of this description was their ponderosity, there being only about 13½ cubic feet of Rubislaw granite to the ton, and 15 feet of Mylnefield stone, while the more common kinds of sandstone contain about 15½ feet to the ton.

These, and other matters of minor importance alluded to in this report, having been adjusted by the Light-house Board, the writer took the necessary measures for entering into contracts and agreements for the supply of stones from these quarries. The difficulties which subsequently attended the procuring of a regular supply of stones for the work have already been alluded to; and to this subject we shall again have occasion to recur, as it was ultimately found necessary to restrict the use of granite to the outward casing of the first thirty feet or solid part of the building.

Mortar of the ancients.

The best composition for building-mortar appears to have been a problem from the earliest history of the arts. Vitruvius, who lived about 130 years before the Christian æra, seems to have been practically, as well as scientifically, acquainted with the whole subject of architecture. But, although he, and other eminent authors who followed him, have minutely treated of the composition of mortar, stating, no doubt, all that was known of the practice of the ancients; yet, it has always been a favourite maxim to maintain, that the secret of compounding mortar has at some period of its history been irrecoverably lost. It is certainly true, that many of the works of ancient times exhibit wonderful specimens of the excellency of their building materials. It may, however, be drawn no less conclusively from the writings of intelligent travellers, that many of their finest edifices have been subject to premature decay, which affords a proof that at least no systematic rule was universally observed in the preparation of their calcareous cements; but that, like the artists of the present day, the quality of their materials depended much upon those adventitious circumstances which too often regulate the views of their successors, by an over-anxious desire for economy, without keeping duly in view the permanency of their works.

Attention of the moderns to this subject.

In Great Britain, the composition of mortar does not seem to have occupied much of the attention of the learned, prior to the beginning of the 18th century, or the time of Sir Christopher Wren. And, indeed, the subject was not pursued with much intelligence and effect, till after the great discoveries of Dr Black, about the year 1754, which unfolded the principles of latent caloric, and the expulsion of fixed air, by which limestone loses about one-half of its weight in the process of calcination. These discoveries were succeeded by the excellent treatise of Dr Higgins on Water Cements, published in 1780; and in 1793, Mr Smeaton’s Narrative of the Edystone Light-house appeared, containing, not only an account of the preparation of the mortar for that celebrated building, but also of his experience for thirty-six years, as an engineer of the most extensive practice of his day. The composition of mortar has also occupied the attention of several French authors, as Belidor, Loriot, Viccat and others, but without perhaps adding much to our stock of practical knowledge.

Experience of the writer.

Were the writer permitted to state the result of his professional observations for the last twenty years, he might notice, that no error is more commonly met with in water buildings, than that of employing house or common mortar in the erection of sea-walls. It may also be stated, generally, in compounding mortar, that the cheapest article is too apt to be made use of in the greatest proportion. We accordingly find, that lime is not unfrequently made too rich, as it is technically termed when a small proportion of sand is applied to the mixture; an error, which is attended even with worse consequences than when the lime is made poor, or when too great a proportion of sand is used. But, perhaps, the worst of all mortar is that wherein very fine pit-sand, containing a portion of earthy matters, is used, and when the whole is mixed up with impure water. So little attention is often paid to the quality of mortar, in common buildings, that one would imagine it were applied, as if intended more to prevent the sifting winds from penetrating the walls, than as the medium by which they were ultimately to be bound or formed into a compact fabric.

It is not possible to give any formula for the composition of mortar which will apply generally; so much depends upon the quality of the limestone, the mode of its treatment in burning, the use of clean sharp sand and pure water. When these ingredients are judiciously selected, duly apportioned, and well beaten together, they will immediately form a paste of some tenacity, which will ultimately take bond and give a consistency to the work. From all the experiments that have been made, it seems to be essential to the composition of the best water-cements, that the limestone should contain about one-seventh of alumine or clayey matter. But as this description of limestone admits of a less proportion of sand in the mortar compounded of it, than that which is more purely calcareous, it is not so much in request for the common operations of building, as being less economical. It may, however, be stated, as a pretty general maxim, that where comparatively pure calcareous matter is met with, at least three measures of clean sharp sand, free of earthy particles, mixed with one of burned lime, in the state of powder, and a due portion of pure water, well beaten together, will form good mortar for common use.

Mortar of the Edystone.